1951: Chapter 16 – General Principles of Fire Control

1. The considerations which govern the methods of applying machine gun fire are:-

(a) The best fire effect on the whole target.

(b) Economy of time and ammunition.

(c) Simplicity and speed.

(d) Safety of our own troops.

The factor of surprise as applied to fire cannot be over-estimated.

Fire control orders must be framed in such a way that all these requirements are met.

The system of fire control laid down in this pamphlet is worked out on the above basis and should be adhered to. Occasionally the situation may not permit the rules given to be carried out in their entirety. Common sense and a knowledge of how the rules are arrived at will enable the best fire effect to be obtained.

Basis of fire control rules

2. Fire effect is desirable as soon as fire is opened or immediately after. Observation of machine gun fire is only possible on certain types of ground and, particularly in European countries, can never be relied on. The opportunity of correcting fire onto the target by observation of strike will seldom occur.

There is no quick and reliable means of determining with accuracy the effect of climatic conditions. Errors, both of direction and elevation, must therefore be expected. The procedure is to define round the target an area allowing for reasonable errors of direction and elevation, and to apply fire over the whole of this area.

The rules of fire control contained in the following chapters are based on the assumption that insufficient observation of strike will be obtained to deduce the exact positions of the beaten zones. Every endeavour, however, must be made to pick up the strike of the bullets and to correct fire accordingly. Whenever sufficient observation of fire for this purpose is possible, the fire control rules should not be adhered to.

Direct or indirect fire

3. The normal method of engaging a target will be by direct fire ie, by laying on the target over the sights. The main asset of direct fire is its extreme flexibility, which enables a succession of targets over a wide arc to be engaged quickly.

The machine gun is capable of firing indirect ie, the gun is laid on an aiming mark, with the elevation required to hit the target obtained and placed on the gun by means of instruments. Indirect fire is employed when it is impossible or inadvisable to occupy a direct fire position, or when shooting from the map.

The main technical advantage of indirect fire is that the necessity for indicating the target to a number of individuals is removed. The laying of the gun is mechanical and is not affected by light or distance.

The disadvantages are the necessity for additional measurements and calculations, and the difficulties of crest clearance owing to the flat trajectory.

Conditions which obstruct the field of view (eg, bad visibility, fog, smoke etc), often arise after a position is occupied. Consequently, when direct fire is to be employed, certain arrangements for indirect fire should be made as soon as time permits. The details of these arrangements can be found in Chapter 13.

The principles and details of fire control set down in this chapter apply equally to direct and indirect fire. As the methods of fire and details of fire discipline are in many instances not the same, direct and indirect fire are treated separately in Part II.

LESSON 93 – CHARACTERISTICS OF MACHINE GUN FIRE

A – INSTRUCTOR’S NOTE

Aim

1. To teach the fire controller the special characteristics of MG Fire and their effect on the employment of machine gun.

8. State that when the charge in the cartridge is fired, a pressure of 19½ tons per square inch is reached. The gases, building up this pressure very rapidly, hit the base of the bullet with a sharp blow and force it through the barrel.

9. During its passage through the barrel, the bullet is given a rotary movement by the rifling of the barrel, which causes the bullet to have a steady flight. The bullet leaves the barrel with a velocity of 2525 feet a second.

Trajectory

10. Explain that when the bullet leaves the barrel, two further forces act on it:-

(a) Air resistance.

(b) Gravity.

As air resistance overcomes the velocity that the bullet has received from the force of the explosion, gravity exerts an increasingly greater effect and causes the bullet to follow a curved path towards the earth. The path of the bullet is called its trajectory (see Fig 12).

The curvature of the trajectory becomes greater the further the bullet travels. The highest point on the trajectory is at a point approximately two-thirds of the range gun-target.

This is known as the “culminating point.”

Burst of fire

11. State that a burst of machine gun fire is normally 25 rounds, though in emergency longer bursts may be used.

Cone of fire

12. Tell the class that owing to the slightly different elevation with which each bullet leaves the gun and owing to the general vibration of the gun on its tripod during firing, a burst of fire forms a pattern in the air, which is elliptical in shape if shown in the vertical plane. This pattern is called a cone of fire (see Fig 13). The majority of the shots are in the centre of the cone.

Beaten zone
13. Explain that when the cone of fire strikes the ground it forms a long cigar-shaped pattern. This pattern is known as the beaten zone. At shorter ranges the beaten zone, due to the flat trajectory, is very long and narrow. As the range increases up to 2000 yards, the length of the beaten zone decreases due to the increased trajectory and the steeper slope of the bullet. Over 2000 yards, the length of the beaten zone increases as small variations in the muzzle velocities of the bullets become more apparent.

The width of the beaten zone increases steadily with the range. The dimensions of beaten zones given in the range tables refer to fire falling on horizontal ground. Should the ground be falling, the beaten zones will be longer. Should be fire be falling on rising ground, the beaten zones will be shorter. The width is constant on all types of ground. See Figs 14 and 15.

Lengths of beaten zones for varying degrees of rising ground are given in the range tables.

Plunging fire

14. The beaten zone will also be shortened if guns are fired on to a target on a lower level than the gun position. This is called plunging fire and should be avoided as it reduces the neutralizing effect of the machine gun.

Effect of trajectory and beaten zone on tactical employment.

15. State that:-

(a) Owing to the flat trajectory of the gun at ranges up to about 600 yards, the machine gun is capable of laying a belt of fire 600 yards long on flat ground, the bullets never rising more than four feet above the ground. This is a valuable asset in defence.

(b) As it is obviously desirable to place the length of the beaten zone along the target when engaging wide targets, machine guns are best sited to a flank where they can employ enfilade fire.

Types of targets

16. Tell the class that the types of targets that machine guns are required to engage are classified as follows:-

(a) Point targets.

(b) Targets with width.

(c) Targets with depth.

(d) Moving targets.

The methods of engaging these types of targets are taught in later lessons.

Conclusion

17. Questions to and from the class.

18. If facilities are available, beaten zones can be demonstrated by firing onto ground or water that will show the strike of the bullets.

19. Sum up main points.

LESSON 94 – ELEVATION
A – INSTRUCTOR’S NOTES

Aim

1. To teach the theory of placing elevation on the machine gun.

Class and instructors

2. One officer instructor and not more than 30 in the class.

Periods

3. One 45-minuted period – lecture.

Stores

4. Blackboard, gun, tripod and dial sight.

Preparation

5. Draw the diagrams shown below on the blackboard. Mount the gun in a position where it can be seen by the class.

B – CONDUCT OF LESSONApproach

6. Explain the aim of lesson (see para 1 above).

7. Emphasize the necessity of being quite clear as to the various angles involved in elevation as they will be constantly referred to when dealing with indirect fire and safety.

Tangent angle

8. Choose a point on the wall on the same level as the gun and tell the class to imagine that the mark is a target at a range of, say, 1000 yards.

Run the slide on the tangent sight up to 1000 and lay the gun on the target by means of the handwheel. Point out to the class that an angle been made between the axis of the barrel and the line of sight through the backsight and foresight to the target. The process of setting the slide and laying the line of sight onto the target sets the axis of the barrel at an angle above the line of sight. This angle is the “tangent angle” for the range at which the target lies (but see para 11).

Tangent angles for all ranges have been determined and are laid down in the range tables.

Confirm that the squad fully understand the tangent angle by revising with the diagram on the blackboard.

Angle of sight

9. Explain that the angle contained between the line of sight to the target and the horizontal plane through the gun position is called the “angle of sight.” This angle is said to be positive (+) when the target is above the horizontal plane and negative (-) when it is below it.

Show the angles of sight in the diagrams below.

Quadrant angle

10. Explain that, when firing indirect, elevation is placed on the gun by means of the dial sight, in two components:-

(a) The range (tangent angle) on the range drum.

(b) The angle of sight to the target on the angle of sight drum.

Explain and demonstrate that if the bubble is then levelled, the elevation to hit the target is placed on the gun. The levelling of the bubble forms a horizontal plane and, as can be seen, an angle has been formed between the axis of the barrel and this horizontal plane. This angle is called the “quadrant angle.” It is used chiefly in recording fixed lines.

Describe, using the diagram below and previous diagrams, how the quadrant angle is composed of the tangent angle and the angle of sight and then it can be calculated from the formula:-

Quadrant angle = tangent sight +/- angle of sight.

Effect of not having a horizontal line of sight

11. Explain that the gun is sighted for a horizontal line of sight. That is to say, if the tangent sight is set at a certain graduation and the gun laid with a horizontal line of sight, a single shot will be theory strike the horizontal plane at a distance away from the gun corresponding to the graduation at which the sight is set.

As the angle of sight increases or decreases, less tangent elevation is required to cause the bullet to travel the same distance, because the force of gravity is not at right angles to lines of sight which are not horizontal.

This point may be more easily understood by the class if the instructor illustrates it by giving the example of firing vertically upward or downward. Here no tangent elevation at all is required on the gun, as the force of gravity acts directly along the line of sight.

Explaing that for angles of sight less than 10 degrees alteration, the alteration in tangent angles required is negligible. In mountainous countries, however, it will be necessary to set the sight at a corrected range. A chart from which the corrected range for abnormal angles of sight can be obtained is given in the range tables.

Conclusion

12. Questions to and from class. The class can be questioned as to the various angles of elevation by drawing diagrams on the board and getting the class to name the angles.

2. To teach the fire controller how to off-set the effect of climatic conditions on elevation and direction.

Class and instructors

3. One officer instructor and not more than 30 in the class.

Periods

4. One 45-minute period, lecture.

Stores

5. Blackboard and chalk. Class will require range tables.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see paras 1 and 2).

Range tables

7. The instructor should explain those parts of the range tables which are described in this lesson and set simple exercises in them, until the class is thoroughly familiar with them.

(a) Pages 2 to 7

Columns 1 and 26 give the ranges in 50’s from 50 to 4,500 yards.

Column 2 gives the tangent angles.

Column 3 gives the lifts for 50 yards, ie, the angular amount by which each elevation has to be increased so as to add 50 yards to the range.

Columns 5 to 13 (see para 9(c) below).

Column 14 gives the number of elevations required by the combined sight rule for the different methods of determining the range (see Lesson 96).

Columns 19 and 20 deal with the cone of fire.

Column 20 gives its total depth, and column 19 gives the angle subtended at the gun by half its depth.

Column 21 gives the width and length of the beaten zone. The figures given are for 90 per cent of the total shots fired. The stray shots, which produce little fire effect, are therefore not included. The length of beaten zone is that along the line of sight.

Column 22 gives the time of flight at each range.

Column 23 gives the slope of descent of the bullets compared with the line of sight. This figure enables a fire controller to calculate whether his fire can be brought to bear on reverse slopes.

Columns 24 and 25 (see para 9(b) below).

Columns 4 and 15 to 18 deal with crest clearance and safety, which will be learnt later.

(b) Pages 14 and 15, give the foreshortening effect of a forward slop on the length of the beaten zone, and the lengthening effect of a reverse slope. The gaps in the bottom left-hand corner of the table are caused by the fact that, at those figures, the reverse slope is steeper than angle of descent of the bullet, with the result that such slopes are “dead ground” when engaged at those ranges.

(c) Page 16 gives the formula to determine the angles of sight and the allowance for moving targets. (Lesson 101).

(d) Page 17. The machine gun is sighted for a horizontal angle of sight, and is therefore sufficiently accurate for all angles of sight between plus 10 degrees and minus 10 degrees.

If the angle of sight exceeds 10 degrees, allowance must be made in accordance with the chart. If it be imagined that a target is being engaged immediately above or below the gun, ie, at an angle of sight of 90 degrees, clearly no tangent angle is required on the sight, no matter what the range. At steep angles of sight, therefore, less elevation is required than for a horizontal angle of sight. (See example at the foot of the chart).

(e) Pages 18 and 19. This table caters for the possible situation in battle where the supply of Mark VIIIZ ammunition has temporarily failed, but where Mark VII is obtainable for filling into belts. Owing to the differences in trajectory of the two kinds of ammunition, the reading on the tangent sight for Mark VIIIZ is not correct for Mark VII. In column 2 is given the reading to be put on the sights when engaging targets shown in column 1.

VI Graph

8. The commonest uses of the VI graphs are:-

(a) Knowing the range, to determine the distance or height subtended by a certain angle.

(b) Knowing the range, to determine the angle subtended by certain distance or height.

In Fig 19 if G is the gun, and GA the range, then AB is the distance subtended by AGB at the range GA, and A1 B1 is the distance subtended by the angle AGB at that range GA1.

Similarly the angle AGB is said to subtend AB and A1 B1 at the ranges GA and GA1 respectively.

Example:-

Angle AGB equals 4 degs 10 mins.

Range GA equals 3,000 yards.

What is the length of AB?

Following the 10 minute column down until it is opposite 3,000 yards read off the amount subtended by 10 minutes at 3,000 yards, ie, 9 yards. Repeat for 4 degrees, ie, 209 yards. Add the two figures together, ie, 218 yards. AB is 218 yards.

9. Climatic influences

(a) The following are the normal conditions for the sighting of small arms:-

Barometric pressure – 30 inches (mean sea level).

Temperature – 60 degrees Fahrenheit.

Still air.

A horizontal line of sight.

(b) Barometer and temperature.- if the barometer falls below 30 inches, less elevation that is normally required for the distance will be necessary, as the atmosphere, being less dense, offers less resistance to the bullet. It should be noted that the barometer will fall one inch for every 1,000 feet above mean sea level. If the barometer rises above 30, inches, more elevation is required, as the air is more dense. The bullet meets with less resistance in hot weather when the temperature is high and the air less dense; and greater resistance in cold weather, when the temperature is low. In the former case, therefore, less elevation is required, and in the latter more.

Allowances for one inch rise or fall of barometer and 10 degrees rise or fall in temperature will be found opposure each range in columns 24 and 25 of pages 2 to 7 of the range table. It should be noted that for a fall in barometer, and in a rise in temperature, the allowance to be made is subtracted and vice versa.

(c) Wind.- Winds blowing directly along the line of fire from front to rear will affect the elevation, but here again unless the wind is very strong and the range long, the allowance required is small.

Winds blowing directly at right angles to the line of fire will affect direction and have considerable effect on the bullet, particularly at long ranges.

Winds blowing from a direction oblique to the line of fire will affect both direction and elevation.

Althought where speed is essential it may be necessary to estimate in taps the lateral allowance to be made for a side wind, reference should be made to the wind table in the range tables when time permits.

Having estimated the strength and direction of the wind, the allowance required may be obtained from the range tables on pages 2 to 7, columns 5 to 13.

An explanation of the use of the wind tables is on page 8 on the range table.

11. Practise the squad in correcting the range to cater for varying climatic conditions at various ranges.

Conclusion

11. Questions from class.

12. Further practice if required.

13. Discuss progress made.

14. Sum up main points.

LESSON 96 – ERRORS IN DIRECTION AND ELEVATION
A – INSTRUCTOR’S NOTES

Aim

1. To explain the causes of errors in direction and elevation and to show the means by which these errors are overcome.

Class and instructors

2. One officer instructor and preferably not more than 30 in the class.

Periods

3. One 45-minute period, lecture.

Stores

4. Blackboard and chalk. Class require range tables.

Preparation

5. Write the combined sight rule on the blackboard.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see para 1 above).

Errors in elevation

7. Errors in elevation may be caused by:-

(a) Inaccuracy in determining the range.

(b) Incorrect allowance for climatic conditions.

(c) Slight inaccuracies of aim or sighting of the guns.

8. The range may be determined by:-

(a) Rangefinder – the most accurate method.

(b) Measurement on a map of not less scale than 1/25,000. The amp must be in good condition and target and gun accurately located.

(c) Key ranging – that is by estimating from ranges taken by either of the above methods. This method is reasonably accurate up to 1,500 yards but beyond that should only be used in emergency.

9. To ensure hitting the target, possible errors in elevation must be allows for. To this end, the combined sight rule has been evolved. Dependent on the method used for determining the range, this rule gives the number of elevations to be employed at all ranges.

COMBINED SIGHT RULE

Range

Number of elevations

0 – 1400

1

1450 – 2000

3

2050 – 2800

5

10. The fire controller should fire at the target with the range on the sights that he has already determined plus or minus any allowance for wind. Should the rule require 3 elevations, he will then order “All down 50, Go on” fire on that elevation and then order “All up 100, Go on”. He was then fired at the supposed range to the target, the range 50 yards below the target and the range 50 yards above the target.

Should the rule require 5 elevations, he will subsequently order “All down 150, Go on” and “All up 200, Go on”. He has thus, in addition, fired with the ranges 100 yards below and 100 yards above the target.

11. If the target has a difference in range to each end, the mean range will be used for determining the number of elevations required to cover possible errors in elevation.

12. If good observation of fire is obtained, the combined sight rule will not be applied.

Errors in direction

Explain:-

13. Errors in direction may be caused by:-

(a) Wrong estimation of the strength of side winds.

(b) Slight inaccuracies of aim.

(c) Wear in the mounting.

14. As the errors may act in either direction, it will be necessary to engage an additional width on either side of the target. Lateral errors will not be great but the beaten zone is narrow and does not give much help in overcoming them. Guns will initially be laid and fired at the target and, if necessary, tapped and fired to cover the width of the target. Then, to ensure that possible errors are covered, arrangements will be made for guns to fire one tap (15 minutes) outside the edges of the target.

15. Owing to the width of the beaten zone, if good observation of fire is obtained the tap to cover errors in direction may be cut out.

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